U.S. Pat. No. 3,842,194 of Clemens discloses an information recording and playback system which utilizes variable capacitance pickup. In one configuration, information representative of picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, a groove width about 2.6 micrometers and groove depth of about 0.5 micrometer may be used. During playback, a pickup stylus, also called a playback stylus, about 2 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove of the disc record as it is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the prerecorded information.
Keizer in U.S. Pat. No. 4,162,510 discloses a novel keel-tipped pickup stylus structure. The keel-tipped stylus comprises a dielectric support element having a body, a constricted terminal portion, and shoulders interconnecting the body with the constricted terminal portion. The electrode surface is remote from the prow end of the stylus.
Other systems have been developed which utilize information disc records which do not contain grooves. Instead, signals are encoded in the disc record surface which guides the pickup stylus during playback.
A problem which can occur during playback using either a grooved or nongrooved disc record is the interference of low frequency signals associated with the audio signals with the video signals. This low frequency interference, called sound beat interference, causes the picture quality to degrade and the picture to break up.
One method of overcoming this problem is disclosed in Clemens et al., U.S. Pat. No. 3,930,117. In the Clemens et al. patent the metal electrode layer on the dielectric support element is overcoated with a dielectric layer which serves to balance the fringing capacitance dielectric path through the dielectric support element. It is believed that imbalance between the path through the dielectric support element and the path through the air in the absence of the dielectric layer contributes to the observed sound beat interference.
In the copending application of Mindel, entitled "Video Disc Playback Stylus," Ser. No. 245,715, filed Mar. 20, 1981, the sound beat problem is also addressed. Mindel oxidizes the outer portion of the conductive metal electrode layer. The oxidized metal layer serves to improve the balance of the dielectric paths.
In the copending application of Rose, entitled "Capacitive Information Systems" Ser. No. 208,983, filed Nov. 21, 1980, and now U.S. Pat. No. 4,371,961 the angle between the electrode surface and the disc record surface is other than 90 degrees in order to balance the dielectric paths between the disc record surface and the electrode, both in the direction of relative motion and in the direction opposite the direction of relative motion.
It would be desirable to have an alternative capacitive pickup information disc playback stylus structure for improving the quality of the signal recovered from the capacitive disc record.